1. Field of the Invention
The present invention relates to a power semiconductor module including bus bars that are conductive members and solder-joined to both sides of a sheet-shaped semiconductor device including a power semiconductor element, such as an IGBT.
2. Description of the Related Art
Widely used power semiconductor elements include IGBT (Insulated Gate Bipolar Transistor), IEGT (Injection Enhanced Transistor), and MOS-FET (Metal Oxide Semiconductor Field Effect Transistor). The power semiconductor element is formed in a sheet-like shape. The power semiconductor element includes a first power terminal and a control terminal on its main surface, and a second power terminal on the back surface thereof. When the power semiconductor element is an IBGT element, the first power terminal is an emitter electrode, the second power terminal is a collector electrode, and the control terminal is a gate electrode.
Jpn. Pat. Appln. KOKAI Publication No. 2003-10064 or 2002-164485 discloses techniques for forming the semiconductor device using such a power semiconductor element mounted on the substrate. According to such techniques, the second power terminal of the semiconductor element is solder-joined to the electrode on the surface side of the substrate, and the first power terminal and control terminal of the semiconductor element are connected to the electrode on the substrate surface using wire bonding with aluminum wire.
In the wire boding, one piece of wire is bonded at a time, thus requiring a lot of bonding time. In the wire bonding, the wire has a loop-like form, thus has a long wire length, resulting in an increased wiring inductance. Further, the technical problems of the wire are that it is sensitive to oscillation, and that there is high possibility of cutting or proximity short circuit.
Thus, provided are some methods: for bonding an aluminum thin sheet to the first power terminal of the semiconductor element; and for solder-joining the first power terminal of the semiconductor element with a flat plate or the lead so as to extract it as an electrode. Particularly, the first power terminal of the semiconductor element is formed of a material usable for solder joints. The method for solder joining the terminal with the flat plate or the lead has come to public attention. The lead-out wire from the control terminal is connected to the terminal by wire bonding.
In the semiconductor module including a plurality of semiconductor devices (hereinafter referred to as “semiconductor packages”), the plurality of semiconductor packages are arranged in line on a base substrate as a heat dissipation plate. Then, the back surface of the substrate of the semiconductor package is joined onto the base substrate. Such a semiconductor module is mounted on a power controller, such as an inverter, converter, etc.
However, in the above-described semiconductor module, only one surface of each semiconductor package is in contact with the base substrate. Thus, sufficient heat dissipation cannot be attained. The substrate back surface of each semiconductor package is joined onto the base substrate, resulting in a large installation area of the semiconductor package with respect to the base substrate and increasing the size of the semiconductor module.
To overcome the above disadvantages, the semiconductor packages are arranged in line, bus bars as conductive members are installed on and sandwiched between the main surface and back surface of the semiconductor package, and these elements are provided on the base substrate.
To be more specific, a solder member is provided for coating at least one of the main surface and back surface of the semiconductor package and the surface of the bus bar where the semiconductor package is provided. The sheet-shaped semiconductor package is set up so as to be in a vertical posture, and the side surfaces of the bus bars are aligned to the main and back surfaces so as to temporarily be held, and the package is contained in a heating furnace and heated therein. The solder member melts so as to solder-join the semiconductor package to the bus bar.
A problem still remains when the semiconductor package is solder-joined to the bus bar through the solder member. That is, when the sheet-shaped semiconductor package is set up in a vertical posture, the main surface and back surface of this semiconductor package and the side surfaces of the solder-joined bus bars will be vertical planes. A part of the solder member which has melted by being heated in the heating furnace flows out through the lower end of the semiconductor package to the lower end of the bus bar, under the influence of gravity. That is, a part of the solder member which has melted by being heated in the heating furnace is likely to spread out to the lower end of the semiconductor package.
In order to prevent the flowing out of the solder, it is considered to lower the amount of solder member. However, in this case, air bubbles are likely to be formed in a solder layer formed between the semiconductor package and the bus bar, thus not attaining a secure and highly reliable solder joint.
Accordingly, when the semiconductor package is set up in a vertical posture so as to be solder-joined to the bus bar, it is difficult to attain an optimum joint thickness of the solder between the semiconductor package and the bus bar, thus resulting in low reliability of the solder joint structure.